Gravure printing and flexographic printing are widely used for the purposes of imparting superior esthetics and functionality to printed items, but in recent years, as a result of the increasing diversity in packages and improved packaging techniques, together with the need to deal with environmental issues from a legal and regulatory perspective, the performance levels required from printing inks become more and more diverse every year. In particular, as a result of the Air Pollution Control Act that came into effect several years ago, the removal of toluene from inks has progressed rapidly, and the maintenance of performance levels in systems from which toluene has been eliminated is a significant challenge, with the entire industry striving to improve printability and printing effects.
Against this background, and for reasons including reducing the amount of residual solvent, improving the suitability for extrusion lamination, and enhancing the solubility in toluene-free solvents composed mainly of ester solvents and/or alcohol solvents, improved polyurethane resins continue to be developed as the binders for all manner of inks, and especially for inks for lamination. The polyurethane used in a typical laminate ink represents 40 to 80% of the ink film, and therefore the selection of the polyester polyol, polyether polyol and diisocyanate used as the polyurethane raw materials and the polyamine used as the chain extender, and the setting of the ratio between these components, have a significant effect on the performance of the printing ink, including the physical properties of the ink film and the printability.
Further, inks for lamination naturally require superior coating film properties such as good laminate strength and adhesion to various printing substrates such as OPP and PET films and synthetic papers, and superior blocking resistance, but also require good printability. A first problem which must be overcome to achieve good printability is the problem of dot reproducibility, which is a phenomenon that occurs when a colored ink is first printed onto a printing substrate, and particularly a film, and a white ink is then overprinted on top of the colored ink, with the problem occurring when the dot of the colored ink is dissolved by the solvent of the white ink. A second problem is the problem of plate clogging, which is a phenomenon that occurs during long print runs when the cells of the gravure cylinder become blocked with insoluble matter, making it impossible to transfer the ink to the printing substrate, particularly in highlight regions.
JP 2010-53194 A (Binder for printing ink, printing ink composition using the same binder, as well as covering material using the same printing ink) describes a combination of favorable coating film properties and printability, but in this document, only plate fogging is mentioned in relation to printability, and the plate clogging resistance and dot reproducibility of the ink can certainly not be claimed to be favorable. Further, although synthesis examples have been reported (JP 2011-144354 A, JP 3,972,373 B) in which, during the step of using an amine as a chain extender in the urethane synthesis, a trifunctional or higher polyamine is selected as the polyfunctional amine used for effecting a chain extension in an aqueous emulsion polymerization system, and the polyfunctional amine is incorporated within a micellized prepolymer in the water, and undergoes polymerization inside the micelles to form an aqueous emulsion resin binder, obtaining a solution-based resin binder that is of practical use as a printing ink binder by using a trifunctional or higher polyamine in a homogenous polymerization system in an organic solvent has proven very difficult.